An example of the arrangement of a conventional digital still camera will be described with reference to FIG. 7.
In FIG. 7, when the photographer operates a camera operation switch 201 (comprised of, e.g., the main switch and release switch of a camera), an overall control CPU 200 detects a change in the state of the camera operation switch 201, and starts supplying power to other circuit blocks.
An object image within the photographing frame range is formed on an image sensing element 204 via main photographing optical systems 202 and 203, and converted into an analog electrical signal. The analog electrical signal from the image sensing element 204 is subjected to analog processing by a CDS/AGC circuit 205, converted into a predetermined signal level, and converted into a digital signal for each pixel by an A/D converter 206.
A driver circuit 207 controls horizontal driving and vertical driving of the image sensing element 204 on the basis of a signal from a timing generator 208 which determines the driving timing of the whole system. The image sensing element 204 then outputs an image signal.
Similarly, the CDS/AGC circuit 205 and A/D converter 206 also operate on the basis of timings from the timing generator 208.
Reference numeral 209 denotes a selector which selects a signal on the basis of a signal from the overall control CPU 200. An output from the A/D converter 206 is input to a memory controller 215 via the selector 209, and all signal outputs are transferred to a frame memory 216. In this case, all pixel data of photographing frames are temporarily stored in the frame memory 216. For sequential shooting or the like, all pixel data of photographed images are written in the frame memory 216.
After the end of write in the frame memory 216, the contents of the frame memory 216 which stores pixel data are transferred to a camera digital signal processor (DSP) 210 via the selector 209 under the control of the memory controller 215. The camera DSP 210 generates R, G, and B color signals on the basis of pixel data of each image stored in the frame memory 216.
Before normal photographing, the generated R, G, and B color signals are periodically (every frame) transferred to a video memory 211, obtaining a viewfinder display or the like by a monitor display 212.
When the photographer designates photographing (i.e., image recording) by operating the camera operation switch 201, pixel data of one frame are read out from the frame memory 216 in accordance with a control signal from the overall control CPU 200, subjected to image processing by the camera DSP 210, and temporarily stored in a work memory 213.
Data in the work memory 213 is compressed by a compression/decompression unit 214 on the basis of a predetermined compression format. The compressed data is stored in an external nonvolatile memory 217 (generally, a nonvolatile memory such as a flash memory is used).
To observe photographed image data, data which is compressed and stored in the external memory 217 is decompressed into normal data of each pixel via the compression/decompression unit 214. The decompressed data of each pixel is transferred to the video memory 211, allowing to observe the photographed image via the monitor display 212.
In this manner, in a general digital camera, an output from the image sensing element 204 is converted into actual image data via the signal processing circuit in almost real time, and the result is output to the memory or monitor circuit.
In such digital camera system, compatibility with a silver halide film such as a 135 format film camera system is important particularly for an interchangeable lens type single-lens reflex camera.
The lens can be used as far as the mount is common. However, the photographing view angle, i.e., focal length suffers a difference in size between the image sensing element and the film.
At present, the size of the image sensing element which can be manufactured at once is limited owing to the manufacturing apparatus, i.e., so-called stepper. Also in terms of cost, the image sensing element is generally smaller than the film. Considering the same photographing sense as that for the film, and particularly photographing with a wide-angle lens, an image sensing element equal in size to a silver halide film is desirable.
As one measure, FIG. 8 schematically shows one image sensing element such as a CCD which is constituted by joining in three-divisional exposure (to be referred to as joint exposure hereinafter).
In FIG. 8, one image sensing element is divided into three, left, center, and right regions. The regions are exposed to individual masks and finally joined into one image sensing element. In FIG. 8, joint exposure is executed in a vertical structure of a semiconductor layer, on-chip color filer layer, and on-chip microlens layer to constitute an image sensing element equal in size to the film.
FIG. 9 is a view for explaining a semiconductor layer when the image sensing element in FIG. 8 is a CCD. In this CCD, the charges of pixels generated in a photodiode 190 are transferred at once to vertical CCDs 191 at a predetermined timing. The charges in the vertical CCDs 191 on all lines are transferred to horizontal CCDs 192, 193, and 194 at the next timing.
In the arrangement shown in FIG. 9, the horizontal CCDs 192, 193, and 194 transfer charges to a common amplifier 195 every transfer clock. An amplified output is read out via common CDS/AGC circuits 196 and 198.
Such CCD can be used similarly to a general image sensing element as far as joint exposure is successful.
Even in the CCD of FIG. 9 which looks like a general image sensing element, a shift by joint exposure actually exists in each vertical structure of a semiconductor layer, on-chip color filter layer, and on-chip microlens layer. The output level varies between the three regions.
Especially, the on-chip color filter layer and on-chip microlens layer readily shift, and the influence appears as a step in the gain direction. In particular, the on-chip color filter layer is exposed for each color, the shift varies, and the step varies between colors.
As shown in FIG. 10, the regions may twist in the plane (two-dimensionally), resulting in a very complicated shift.